The potential of various materials for use on reusable space transport vehicles with sharp leading edges is being investigated. The sharp leading edges (e.g., at the nose and wing edges) of these vehicles will experience significantly higher heating (to greater than 2000° C.) during hypersonic flight compared to the heating (˜1650° C.) at the relatively blunt leading edges of the current space shuttle orbiters. This requires the development of new materials that are capable of reliably withstanding these high temperatures, including resisting oxidation and maintaining high mechanical strength at such temperatures, as well as withstanding the stresses from large spatial temperature differentials or sudden changes in temperature (improved thermal shock properties). A high thermal conductivity is also preferable, since it allows the heat energy to conduct through the material away from the hottest zones and to be re-radiated through cooler surfaces.
Ultra high temperature ceramics (UHTC), which are composed primarily of metal borides, carbides and nitrides, oxides or silicides, and especially of refractory metal diborides, are candidate materials for the sharp leading edges on hypersonic re-entry vehicles. UHTCs are a family of ceramic materials with very high melting temperatures and reasonable oxidation resistance in re-entry environments. Ground based arc-jet testing has demonstrated their potential for applications at temperatures approaching 2200° C.
However, there is concern regarding the mechanical properties and reusability of UHTC materials, in particular their low thermal shock resistance and low fracture toughness (resistance to crack propagation). Monolithic UHTCs are of concern because of their low fracture toughness and brittle behavior, leading to the possibility of sudden catastrophic failure of the material in the extreme re-entry environment. Future generation materials for use on space transport vehicles require substantial improvements in material properties, leading to increased reliability and safety. It is yet to be determined if UHTCs can be made to possess the properties necessary to reliably withstand the extreme environments experienced at the leading edges during re-entry without undergoing some recession, oxidation or thermal shock.
UHTC composites are being investigated as a possible approach to overcome the mechanical deficiencies of monolithic UHTCs. Mechanisms responsible for enhanced toughness in ceramic composites include crack deflection, crack bridging and microcracking. It is also known that high aspect ratio microstructures can lead to enhanced performance. The mechanical performance of ceramics in general would benefit from a high aspect ratio reinforcement phase. Previous work in other ceramic systems has shown that these mechanisms can result in a fracture toughness of 10 to 30 MPa√m, compared to typical values of only 2 to 6 MPa√m in monolithic materials.
A small grain size, high aspect ratio, uniform distribution and random orientation of the reinforcing microstructures are very important for attaining the best performance from a ceramic composite. Mixing the reinforcement phase in powder form with the matrix phase (also in powder form) prior to thermal consolidation sacrifices each of these desired characteristics to some extent, as the resulting microstructure of the consolidated material has larger and rounder grains than desirable, sufficiently uniform mixing is difficult to achieve.
Early efforts with hot pressed HfB2: 20 vol % SiC focused mainly on improving homogeneity in the resulting microstructure and on characterizing its baseline properties. The SiC reinforcement material was found to promote refinement of the microstructure in comparison with monolithic HfB2 material, but also to decrease the thermal conductivity. Additionally, more SiC was not necessarily better from an oxidation standpoint.
U.S. Pat. No. 6,146,559 to Zank describes preparing titanium diboride ceramics by mixing titanium diboride powder with a pre-ceramic organosilicon polymer, then molding and sintering under pressure to achieve high density in these systems.
In NASA Technical Memo NASA/TM-2004-213085, S. Levine et al., “Characterization of an Ultra-High Temperature Ceramic Composite” (NASA Glenn Research Center, Cleveland Ohio, May 2004), a UHTC composite plate was produced from eleven plies of carbon fabric alternately coated with a SiC/AHPCS (allylhydrido-polycarbosilane) slurry and a HfB2/AHPCS slurry. The coated cloth was pressed in a mold to 12 MPa, cured under inert gas to 400° C., then fired to 850° C. under inert gas to pyrolyse the AHPCS.
Kuntz et al., “Properties and Microstructure of Alumina-Niobium Nanocomposites Made by Novel Processing Methods”, Ultrafine Grained Materials II, TMS 2002, pp. 225-234, indicates that the addition of refractory metals, such as niobium, to ceramic structures made by spark plasma sintering can more than double fracture toughness to greater than 6 MPa√m with only marginal decrease in hardness.
U.S. Pat. No. 6,287,714 to Xiao et al., a WC/Co cermet system has a BN grain growth inhibitor incorporated by spray drying a poly-urea-boron precursor onto the WC powder prior to densification. The resulting nanostructure material has increased toughness to 15-30 MPa√m with the grain growth inhibitor added.
An effective way to incorporate the microstructures into the ceramic composite is needed if optimum fracture toughness is to be attained.